1. When I built my original main addon frame, I forgot to make it a child of UIParent. Now, when I put parent="UIParent" into the frame tag, my entire addon frame becomes very tiny.

    Why is this?

    Because the effective scale of your frame is the product of the scale of your frame and the effective scale of your parent's frame. This means if your UI scale is set to something low it will make your frames smaller. If you use scaling in your own addon, this will be compounded. You can easily adjust all of this.

    OK, so three further questions come to mind:

    1. Am I better off leaving it bound to UIParent and adding some scaling?

    2. Am I better off leaving it bound to UIParent and re-adjusting all my sizing / dimensions so it looks right on my screen now?

    3. If I do not bind it to UIParent at all, how will this affect how others see my addon on their screens?

    Thanks again.

    OK, so three further questions come to mind:

    1. Am I better off leaving it bound to UIParent and adding some scaling?

    You shouldn't be using scaling in the first place, imo (and you may not be).

    1. Am I better off leaving it bound to UIParent and re-adjusting all my sizing / dimensions so it looks right on my screen now?

    Your frame should be parented to UIParent and should be a particular size that can be configured by the user (perhaps by adjusting scale from the default of 1.0). This allows them to make things bigger and smaller as they'd like, but will match the default user interface.

    1. If I do not bind it to UIParent at all, how will this affect how others see my addon on their screens?

    Alt-Z won't hide your frames, and I'm sure there are other bits. It's just good practice and you should always try to do it (UIScale being the perfect reason why).

    OK. I'll bind it to UIParent and then readjust my size numbers.

    Thanks again.